The invention relates to a control device for positioning an instrumented cable provided with buoyancy means for retrieving the control device and the instrumented cable from submerged position. Especially the invention is related to a control device provided with buoyancy means for buoyantly lifting the control device and the instrumented cable to the surface.
A seismic instrumented cable (streamer) is an elongate cable like structure (often up to several thousand meters long), which comprises an array of hydrophone cables and associated with electric equipment along its length, and which is used in marine seismic surveying. In order to perform a 3D/4D marine seismic survey, a plurality of such instrumented cables is towed behind a seismic survey vessel. Acoustic signals produced by the seismic sources are directed down through the water and into the seabed beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones, and next digitized and processed to build up a representation of the earth strata in the area being surveyed.
The instrumented cables are typically towed at a constant depth of about five to ten meters, in order to facilitate the removal of undesired “false” reflections from the water surface. In order to keep the instrumented cables at a constant depth, control devices known as “birds” are attached to each instrumented cable at intervals of 200 to 300 meters.
There exist many types of control devices, e.g. as described in U.S. Pat. No. 6,011,752 and U.S. Pat. No. 6,671,223. Other examples of birds are NO20092577, NO20092575, NO20100256, NO20080145 and NO20064102 all in the name of the applicant.
The control devices are complicated and expensive control units that are working under severe conditions regarding stress and harsh submarine conditions. They may collide with objects in the water or get entangled in lost fishing lines, fishing nets or other objects in the sea, thereby putting the whole seismic operation in jeopardy. A dysfunctional control device may take uncontrolled paths and collide with other instrumented elements leading to a worst case scenario of terminating the whole operation. A dysfunctional control device may also lead to entangling of several instrumented cables, thereby creating a real disaster for the seismic operation.
Naturally, a control device with operational problems will need to be serviced as fast as possible. This can be done by forcing the control device to the surface where it can be serviced by a workboat. This method of servicing will be much faster and much cheaper than reeling the whole instrumented cable on board in order to reach the control device in question.
There exist several different solutions for retrieval of instrumented cable/streamer which also will retrieve the control device to the surface together with the instrumented cable/streamer.
From U.S. Pat. No. 4,823,325 it is known a seismic streamer retrieval system which utilizes an inflatable bag to lift the streamer to the water surface where it can easily be recovered. An acoustical signal or a pressure switch is used to activate said device so as to cause the inflatable bag to be ejected from said device and inflated to lift the streamer to the water surface. A small electric motor and drill bit are used to puncture a gas cylinder to release gas into the inflatable bag. U.S. Pat. No. 4,823,325 describes a device which can be attached to the streamer by means of collar assemblies, accordingly, this is not a device arranged in-line the instrumented cable. Retrieval devices in accordance with this design suffer from a number of disadvantages. As the retrieval device is arranged exterior of the instrumented cable, and e.g. hang beneath the instrumented cable, they produce considerable noise as they are towed through the water, which noise will interfere with the reflected signals detected by the hydrophones in the instrumented cable. This solution will also be highly vulnerable in underwater operations, and will be exposed to damages by objects in the water. Also, the need of having several such devices arranged along the instrumented cable will complicate the retrieval and deployment of the instrumented cable from the vessel, as these device will have to be removed before the instrumented cable can be reeled onto a drum at the vessel, and will have to be arranged to the instrumented cable after the instrumented cable leaves the drum to be deployed into the water.
From U.S. Pat. No. 6,612,886 it is known an in-line cable retriever, which solves many of the mentioned problems with U.S. Pat. No. 4,823,325. U.S. Pat. No. 6,612,886 describes a retriever device for a marine seismic cable or other tool. The retriever device comprises a housing, in which housing is disposed a canister for containing gas under pressure. An inflatable bag is fluidly connected to the gas canister within the housing. Gas is prohibited from exiting the gas canister and inflating the bag until hydrostatic pressure acting upon the retriever device exceeds a designated amount. If the cable sinks to a certain depth within a water body, the corresponding hydrostatic pressure acting upon the cable will exceed a preset pressure level. An actuating mechanism is then actuated, causing gas to exit the gas canister and to inflate the inflatable bag. In one arrangement, a pop-open cover is provided on the housing. As the bag inflates, it acts to release the pop-open cover from the housing and to further inflate outside of the housing of the retriever. The inflated bag will cause the cable to be buoyantly lifted to the surface of the water body.
Even though U.S. Pat. No. 6,612,886 solves the problem of noise in relation to U.S. Pat. No. 4,823,325, it also introduces new problems. First of all, it is not arranged for remote controlling, resulting in that one is completely reliable on the pressure sensor for activation. Secondly, by the lack of remote control it will not be possible to bring the cable to the surface for maintenance of e.g. control devices with failure. Thirdly, the fact that this retriever device is arranged in-line the instrumented cable introduces increased risk of failure in the instrumented cable. The more connections, the more is the risk for leakage and damage of the instrumented cable. By that this retriever device is arranged in-line the instrumented cable, this also results in that the retriever device must be provided with means for continuing the electrical cables of the instrumented cable between the sections, accordingly introducing two new connection points for each retriever device.
Accordingly, this solution introduces a higher risk of instrumented cable failure due to e.g. leakage, and this solution will increase the costs due to the fact that one will need a number of such devices along an instrumented cable.
There exists today no control device for instrumented cables being provided with buoyancy means for buoyantly lifting the control device and instrumented cable to the surface if desired or needed.
Accordingly there is a need for a more operational viable and cost effective solution to retrieving a control device and instrumented cable (segment) to the surface for inspection and servicing.
There is also a need for a solution which makes it possible to exchange control device while the instrumented cable is still in the sea.
There is also a need for a solution of retrieving which does not increase the risk of danger for failure of the instrumented cable or control devices.